The present invention relates to a method and system for the continuous logging of the gravity gradient along an earth formation. In applications of formation gravity measurements, it is only the gravity gradient that is of interest since the gradient is related to the bulk density of the formation between the measurement stations according to well-known formulas. From this bulk density, estimates can be obtained of porosity and fluid saturation in the porous rocks of the formation. This gravity gradient has been determined by measuring the gravity difference of any two points along the formation and then dividing by the vertical distance separating the two points.
Current gravity technology measures gravity at discrete points along a formation by detecting motion of a small test mass in a gravity logging tool as the tool remains stationary. Small changes in gravitational acceleration cause this mass to move against the restoring force of a spring. Various approaches to the detection of this minute movement are possible. One such method is illustrated in detail in U.S. Pat. No. 4,399,693, to L.S. Gournay. The general arrangement of a gravimetric exploration operation as well as a conventional LaCoste- and Romberg-type gravity meter of the mass-spring combination is shown in such Gournay patent. Also in the Gournay patent is the standard formula for computing formation bulk density from the gravity difference measurements. These gravity difference measurements must be made when the tool is motionless with respect to the formation. Such tool is not practical for making continuous measurements since there is no accurate means for eliminating the force on the test mass due to tool acceleration, and consequently, accurate gravity readings cannot be made with the tool in motion. This limitation requires that the gravity tool be stopped for a period of several minutes at each point or station along the formation at which a gravity difference measurement is desired. Another limitation is that the resolution of the tool is only about ten feet.
It is, therefore, an object of the present invention to provide a method and system for the continuous gravity logging which directly measures the variation of gravity gradient along a formation rather than gravity difference and which is insensitive to the effects of accelerations due to unsteady motions of the gravity logging tool.
In U.S. Pat. No. 4,602,508 of J.L. Fitch and W.D. Lyle, Jr., filed Oct. 4, 1984, there is described a method and system for conducting a continuous gravimetry survey of earth formations. The formation is continuously traversed with a gravity logging tool having a column of fluid within the tool. A first pressure difference is measured along a first interval within the column of fluid. A second pressure difference is measured along a second interval within the column of fluid. These first and second pressure difference measurements are differenced to determine the gravity gradient between such first and second intervals. The first interval over which the first pressure difference is measured lies within a first portion of the logging tool and the second interval over which the second pressure difference is measured lies within a second portion of the logging tool. In an alternate embodiment, the first and second intervals overlap at least along a portion of the fluid column. The logging tool includes a container for the fluid column which maintains a constant pressure along the length of the column. Temperature is maintained constant along the column by an external heat absorber or by a heat absorbing element located with the column itself. Baffles are located within the fluid column to prevent thermally driven convection. A sound absorber is also located within the fluid column to inhibit the transmission of acoustic waves. The fluid column is further shock-mounted to prevent acoustic waves from being excited within the column itself.